The question is if they ALL want beer. A logician would take that literally and not answer for himself, like a normal person, but treat the question as one about the entire group. Not knowing a priori (one assumes) what 2 and 3 will want, he can't answer yes and be certain, even if he himself wants a beer. He must want a beer himself, or he could answer no, as at least one person will not have one. Etc
Discussion
so what information does the 3rd guy gain through the dialogue and use to make his conclusion?
He deduces both 1 and 2 must want a beer. Fact he can then answer yes means he also wants a beer and can confidently assert they must ALL want beer, the waiter's question.
Perhaps this varient will help clarify:
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